Since the discovery by the University of Dundee Group in the early 1970's that high quality, low density of states amorphous silicon could be produced from the decomposition of silane (SiH4) gas in a glow discharge, amorphous silicon semiconducting devices have emerged as a dominant force in the marketplace.
Amorphous silicon's initial attraction as an electric power source was that it can be fabricated over large area substrates as required by photovoltaic and thin film transistor applications. Large area substrates on which amorphous silicon is deposited include glass plates, large plastic sheets, flexible non-conductive substrates such as plastic ribbons, and flexible metal foil substrates, such as stainless steel.
In order to perform the aforementioned depositions at a commercially viable rate, the industry realizes that a continuous production system must be utilized. However, large area substrates suffer from decreased conversion efficiency, and increase the difficulty by which continuous processes may be employed to produce amorphous silicon semiconducting devices.
One way to sharply reduce the panel cost of amorphous silicon solar cells is to capitalize on the fact that series-connected cells covering large areas can be manufactured as monolithic devices. Because glass substrates are insulating, a monolithic series-connected panel can be constructed with ease.
The fabrication of a complete photovoltaic panel requires a multi-step process integrating many established technologies. Obtaining acceptable panel cost ultimately depends on reducing processing costs, improving yield and increasing throughput at virtually every stage, with reduced pin hole formation a priority.
The most conventional technique for the production of amorphous silicon cells is the "batch" type process which is inherently slow and suffers from sever cross-contamination due to boron and phosphorous. This leads to a system which yields low throughput as well as low over all panel performance. Consequently, the cost of the panels is high. While the "batch" type process was viable in the late 1970's, it is no longer a state-of-the-art approach to mass production.
In the "roll-to-roll" approach, two types of substrates are under investigation--polyamide and stainless steel. These substrates could have major drawbacks for several reasons. Under the "roll-to-roll" approach, a roll of material is fed through the system, so that P, I, and N-layers are deposited sequentially. This, by necessity, results in cross-contamination due to boron (from the P-layer deposition section) and phosphorous (from the N-layer deposition section) and leads to degradation in the opto-electronic properties of the I-layer and consequently in the device-panel performance.
Although various schemes exist to minimize cross-contamination, it should be recognized that even one part per million of boron (or phosphorous) severely affects the properties of the I-layer.
The "roll-to-roll" and the "batch" approaches either lead to a low throughput or cross-contamination problems, leading to low performance cells. The cell performance is inexplicably linked to the density of localized states which are in turn related to impurities and cross-contamination due to boron and phosphorous. It has been found that to obtain high performance cells with reproduceability, one of the key factors is a multi-chamber approach.
U.S. Pat. 4,492,605 issued to Ishihara et al. on Jan. 8, 1985 discloses a continuous in-line deposition system for coating large area substrates with amorphous silicon. However, the deposition of amorphous silicon, as taught by Ishihara, is merely incidental to the movement of the substrate through the system. U.S. Pat. No. 4,318,938 issued to Barnett et al. on Mar. 9, 1982, discloses a technique for manufacturing solar cells by a continuous process suitable for large-scale manufacture which involves providing a reel of thin metal foil substrate and forming on the substrate a series of layers operative to form a photovoltaic junction, short prevention blocking layers, contacts and integral encapsulation. A collector of cadmium sulfide deposited with an absorber-generator of copper sulfide is described.
U.S. Pat. No. 4,405,435 issued to Tateishi et al. on Sept. 20, 1983 discloses an apparatus for performing continuous treatment in vacuum including an inlet chamber, a first intermediate chamber, at least one vacuum-treating chamber, a second intermediate chamber and a withdrawing chamber arranged in a sequential order in a direction in which base plates are successively transferred. A conveyor device for conveying each base plate in a horizontal direction through an opening device is mounted in each of the chambers and an evacuating device is also mounted in each chamber. The disclosure is specific to a continuous sputtering apparatus.
U.S. Pat. No. 4,438,723 issued to Cannella et al. on Mar. 27, 1984 discloses a multiple chamber deposition and isolation system and method including a system to form a body of material on a substrate having at least two layers of different composition with minimized cross-contamination between the respective deposition environments in which the layers are deposited. A continuous flexible metal foil substrate is processed through the multiple chamber system.
U.S Pat. No. 4,593,644 issued to Hanak on June 10, 1986 discloses a continuous in-line deposition system having a vacuum chamber for coating large substrates. The apparatus also includes load lock chambers for entry of substrates supported by carriers into the vacuum chamber and for subsequent exiting after the coating. The carriers transport pairs of substrates with their principal faces held in a plane that is both parallel to the electric field of the glow discharge reaction and perpendicular to the direction of motion of the substrates through the apparatus. A transportation means is included for holding the substrate so that a principal face of each is perpendicular to the longitudinal axis and for imparting continuous motion to the substrate along the longitudinal axis and through each of the subchambers.